Custom

Eleventy now allows the addition of custom template extensions, meaning that you can use Eleventy to process any arbitrary file extension and compile it to your site’s output folder. This feature is .

Introductory Example: *.clowd

clowd is a pretend templating language that we’ve just created. It uses the .clowd file extension. The purpose of the language is to translate any occurrences of the word cloud to the word butt instead.

export default function (eleventyConfig) {
	// Add as a valid extension to process
	// Alternatively, add this to the list of formats you pass to the `--formats` CLI argument
	eleventyConfig.addTemplateFormats("clowd");

	// "clowd" here means that the extension will apply to any .clowd file
	eleventyConfig.addExtension("clowd", {
		compile: async (inputContent) => {
			// Replace any instances of cloud with butt
			let output = inputContent.replace(/cloud/gi, "butt");

			return async () => {
				return output;
			};
		},
	});
};

module.exports = function (eleventyConfig) {
	// Add as a valid extension to process
	// Alternatively, add this to the list of formats you pass to the `--formats` CLI argument
	eleventyConfig.addTemplateFormats("clowd");

	// "clowd" here means that the extension will apply to any .clowd file
	eleventyConfig.addExtension("clowd", {
		compile: async (inputContent) => {
			// Replace any instances of cloud with butt
			let output = inputContent.replace(/cloud/gi, "butt");

			return async () => {
				return output;
			};
		},
	});
};

Example: Add Sass support to Eleventy

For a more realistic sample, here’s an example of Eleventy looking for all .scss files in a project’s input directory to process them to your output directory.

// Don’t forget to `npm install sass`!
import sass from "sass";

export default function (eleventyConfig) {
	eleventyConfig.addTemplateFormats("scss");

	// Creates the extension for use
	eleventyConfig.addExtension("scss", {
		outputFileExtension: "css", // optional, default: "html"

		// `compile` is called once per .scss file in the input directory
		compile: async function (inputContent) {
			let result = sass.compileString(inputContent);

			// This is the render function, `data` is the full data cascade
			return async (data) => {
				return result.css;
			};
		},
	});
};

// Don’t forget to `npm install sass`!
const sass = require("sass");

module.exports = function (eleventyConfig) {
	eleventyConfig.addTemplateFormats("scss");

	// Creates the extension for use
	eleventyConfig.addExtension("scss", {
		outputFileExtension: "css", // optional, default: "html"

		// `compile` is called once per .scss file in the input directory
		compile: async function (inputContent) {
			let result = sass.compileString(inputContent);

			// This is the render function, `data` is the full data cascade
			return async (data) => {
				return result.css;
			};
		},
	});
};

We’re using compileString from the Sass library above for speed benefits over their asynchronous counterparts (reported by the Sass documentation).

Note also that the data is not used in the above example. This is the full Eleventy data cascade and may be more useful in other templating languages.

The above extension would process a file located at subdir/test.scss to the output directory at _site/subdir/test.css.

Using inputPath

You can pass in both the file’s inputPath and the Eleventy includes folder to provide a set of directories to look for when using Sass’ @use, @forward, and @import features. Read more about loadPaths on the Sass documentation.

		// some configuration truncated …
    compile: function (inputContent, inputPath) {
      let parsed = path.parse(inputPath);

      let result = sass.compileString(inputContent, {
        loadPaths: [
          parsed.dir || ".",
          this.config.dir.includes
        ]
      });

      return (data) => {
        return result.css;
      };
    }

Make special note of the this.config.dir.includes folder above. Declaring your includes folder means that you don’t need to prefix any file paths with the includes folder name (e.g. _includes/_code.scss can be consumed with @use "code").

Registering Dependencies Added in v2.0.0

Eleventy includes two features to improve the performance of custom template compilation:

1. A compilation cache, which you can optionally disable with compileOptions.cache
2. Hooks for incremental builds (via the --incremental command line flag)

To facilitate these features, if a template syntax allows use of other templates (think @use in Sass or webc:import in WebC), Eleventy needs to know about the dependencies a template file relies on. This is heavily dependent on each template compiler.

In our Sass example, this is exposed by Sass via the loadedUrls property from the compileString function, and you can see an example of how we register our dependencies in the compile method below:

    // some configuration truncated …
    compile: function (inputContent, inputPath) {
      let result = sass.compileString(inputContent);

      this.addDependencies(inputPath, result.loadedUrls);

      return async (data) => {
        return result.css;
      };
    }

addDependencies’s first parameter is the parent template file path. The second parameter is an Array of child file paths used by the template. The dependencies can be either relative or absolute paths and we will normalize them as needed.

Skipping a template from inside of the compile function

To add support for Sass’ underscore convention (file names that start with an underscore aren’t written to the output directory), just return early in the compile function (don’t return a render function).

    // some configuration truncated …
    compile: async function (inputContent, inputPath) {
      let parsed = path.parse(inputPath);
      if(parsed.name.startsWith("_")) {
        return;
      }

      let result = sass.compileString(inputContent);

      return async (data) => {
        return result.css;
      };
    }

Note that files inside of the _includes folder are left out of processing by default, so if you store your sass @use, @forward, and @import files in there you’ll get this for free (see the Using inputPath example above)!

This functionality is more-or-less identical to the compileOptions permalink: false overrides, documented later on this page.

Aliasing an Existing Template Language

Added in v2.0.0 If key is the only property in the options object, we treat the extension as an alias and use the existing upstream template syntax.

export default function (eleventyConfig) {
	eleventyConfig.addExtension("11ty.jsx", {
		key: "11ty.js",
	});

	// Or, you can pass an array of extensions in v2.0.0 or newer.
	eleventyConfig.addExtension(["11ty.jsx", "11ty.ts", "11ty.tsx"], {
		key: "11ty.js",
	});
};

module.exports = function (eleventyConfig) {
	eleventyConfig.addExtension("11ty.jsx", {
		key: "11ty.js",
	});

	// Or, you can pass an array of extensions in v2.0.0 or newer.
	eleventyConfig.addExtension(["11ty.jsx", "11ty.ts", "11ty.tsx"], {
		key: "11ty.js",
	});
};

You can read about the above approach (and see more detailed examples of its usage) on the TypeScript, JSX, or MDX documentation pages.

Added in v3.0.0 key needn’t be the only property in the options object. If you want to add your own compile function, keep reading!

Added in v3.0.0 Breaking Change: Starting in Eleventy 3.0 you must add the new alias to your declared template formats for the new template type to be processed.

Overriding or Extending an Existing Template Language

You can override or extend existing template languages too! (Thank you to Ben Holmes for this contribution).

In these example, we switch from the Eleventy default markdown-it to marked for markdown processing.

import { marked } from "marked";

export default function (eleventyConfig) {
	eleventyConfig.addExtension("md", {
		compile: function (inputContent, inputPath) {
			let html = marked.parse(inputContent);

			return function (data) {
				// Example: use `marked` only if useMarked is set in the Data Cascade
				if (data.useMarked) {
					return html;
				}

				// You can also access the default `markdown-it` renderer here:
				return this.defaultRenderer(data);
			};
		},
	});
};

const { marked } = require("marked");

module.exports = function (eleventyConfig) {
	eleventyConfig.addExtension("md", {
		compile: function (inputContent, inputPath) {
			let html = marked.parse(inputContent);

			return function (data) {
				// Example: use `marked` only if useMarked is set in the Data Cascade
				if (data.useMarked) {
					return html;
				}

				// You can also access the default `markdown-it` renderer here:
				return this.defaultRenderer(data);
			};
		},
	});
};

Note that overriding md opts-out of the default pre-processing by another template language Markdown Files. As mentioned elsewhere, improvements to add additional hooks for preprocessing will likely come later.

You can override a template language once. Any attempts to override an more than once via addExtension will throw an error.

Added in v3.0.0 Adding key in the options object unlocks use of the target defaultRenderer. You can read about this approach (and see examples of its usage) on the TypeScript, JSX, or MDX documentation pages (all of which use key: "11ty.js" to extend JavaScript templates).

Access to Existing Filters and Shortcodes

If you want to add support for universal filters and shortcodes in your custom template language, you can do so with the following configuration API methods. Related GitHub #3310.

• eleventyConfig.getFilter(name)
• eleventyConfig.getFilters() Added in v3.0.0
• eleventyConfig.getShortcode(name) Added in v3.0.0
• eleventyConfig.getShortcodes() Added in v3.0.0
• eleventyConfig.getPairedShortcode(name) Added in v3.0.0
• eleventyConfig.getPairedShortcodes() Added in v3.0.0

Full Options List

compile

• Required for new file extensions. Optional for aliases.

compile is an async-friendly function that takes two parameters:

• inputContent: the full content of the file to parse (as a string).
• inputPath: the path to the file (as a string, useful for looking up relative imports)

compile can return:

• nothing (undefined) to indicate that the file should be ignored and not used as a page
• a render function (also async-friendly)

	// some configuration truncated …
	compile: async (inputContent, inputPath) => {
		return async () => {
			return inputContent;
		};
	},

The render function is passed the merged data object (i.e. the full Data Cascade available inside templates). The render function returned from compile is called once per output file generated (one for basic templates and more for paginated templates).

	compile: function(compileFn) {
		return function(data) {
			// Binds this.page and this.eleventy to your render context (and any future additions added later)
			let renderFn = eleventyConfig.augmentFunctionContext(compileFn, {
				source: data,

				// Overwrite existing values?
				// overwrite: true,

				// Lazily fetch the key using `getter`
				// lazy: false,
				// getter: (key, context) => context?.[key];
			});

			return renderFn(data);
		};
	}

outputFileExtension

• Optional: Defaults to html

When the output file is written to the file system, what file extension should be used?

init

• Optional

An async-friendly function that runs once (no matter how many files use the extension) for any additional setup at the beginning before any compilation or rendering.

Note that init will not re-run on watch/serve mode. If you’d like something that runs before every build, use the eleventy.before event.

	// some configuration truncated …
  init: async function() {
    // has access to current configuration settings in `this.config`
  },

read

• Optional: Defaults to true

Set to false to opt out of reading the contents of files from the file system. This is useful if you’re using an external bundler to read the files.

	// some configuration truncated …
  read: false,

Use with compileOptions.setCacheKey to get more fine-grained control over how the template is cached.

useLayouts Added in v3.0.0

• Optional: Defaults to true

Whether or not Layouts will be applied to this template language. This will also exclude data from layout files to play a part in the data cascade of this template type as well. Related GitHub #2830.

useJavaScriptImport Added in v3.0.0

Use the JavaScript loader instead of reading from the file system. If enabled, this takes precedence over read option.

	useJavaScriptImport: true,
	getInstanceFromInputPath: async function(inputPath) {
		let mod = await import(inputPath);
		return mod.default;
	},
	compile: (compileFn) => compileFn,

getData and getInstanceFromInputPath

• Optional

Controls if and how additional data should be retrieved from a JavaScript object to populate the Data Cascade. If your templates aren’t compiling JavaScript objects, you probably won’t need this.

Notably, this is separate from (in addition to) front matter parsing (which requires read: true).

	// some configuration truncated …
	// `false` is the default
	getData: false, // no additional data is used

	// some configuration truncated …
  getData: async function(inputPath) {
    // DIY, this object will be merged into data cascade
    return {};
  },

	// some configuration truncated …
  // get the `data` property from the instance.
  getData: ["data"],
  // * `getData: true` is aliased to ["data"]
  // * You can use more than one property name! ["data", "otherPropName"]

  getInstanceFromInputPath: function(inputPath) {
    // Return the JavaScript object from which the `data` property will be retrieved.
    let instance = doSomethingMyselfToFetchAJavaScriptObject(inputPath);
    return instance;
  }

	// some configuration truncated …
  // if getData is `false`, `eleventyDataKey` will not be used.
  getData: true,

  getInstanceFromInputPath: function(inputPath) {
    return {
      // Overrides `getData` for this instance
      eleventyDataKey: ["myOverrideData"],

      // Will not be used
      data: {
        notAvailableOnGlobalData: 456
      },

      // Will be used.
      myOverrideData: {
        availableOnGlobalData: 123
      }
    }
  },

compileOptions

compileOptions.permalink to Override Permalink Compilation

Optional. This has the same signature as the compile function and expects a reusable render function to be returned.

  // some configuration truncated …
  compileOptions: {
    permalink: function(contents, inputPath) {
      return (data) => {
        // Return a string to override: you’ll want to use `data.page`
        // Or `return;` (return undefined) to fallback to default behavior
      }
    }
  },

• Don’t compile permalink strings in the parent template language
  • permalink: "raw" (new default in v3.0, related GitHub #2780)
• Don’t write any files to the file system:
  • permalink: false
  • permalink: (contents, inputPath) => false
  • permalink: (contents, inputPath) => ((data) => false)
• Override the default permalink function (return a string to override)
  • permalink: (contents, inputPath) => "…"
  • permalink: (contents, inputPath) => ((data) => "…") (use the data cascade)
  • If you return nothing (or undefined), this will revert to the default permalink behavior.

This provides another way to implement Sass’ underscore convention to skip writing the file to the output directory:

	// … some configuration truncated
  compileOptions: {
    permalink: function(contents, inputPath) {
      let parsed = path.parse(inputPath);
      if(parsed.name.startsWith("_")) {
        return false;
      }
    }
  },

compileOptions.spiderJavaScriptDependencies

• Optional: Defaults to false

Enable to use Eleventy to spider and watch files require’d in these templates. This allows you to control the Watch JavaScript Dependencies feature on a per-template language basis. Most template languages will want the default here and keep this feature disabled.

compileOptions.cache for advanced control of caching

• Optional: Defaults to the value of read

This controls caching for the compilation step and saves the compiled template function for reuse. For more efficient cleanup (and long term memory use), these caches are now segmented by inputPath (Added in v2.0.0).

By default, whether or not this cache is enabled is tied to boolean value of read. If read: true, then cache will also be true. It’s unlikely you will need this, but you can override this to mismatch read.

You can also granularly control the caching key using a getCacheKey callback. It might be useful to change this when using read: false and contents are unavailable.

	// some configuration truncated …
  read: false,
  compileOptions: {
    cache: true,
    getCacheKey: function(contents, inputPath) {
      // return contents; // this is the default in 1.0

      // return inputPath + contents; // this is the new default in v2.0.0

      return inputPath; // override to cache by inputPath (this means the compile function will not get called when the file contents change)

      // Conditionally opt-out of cache with `return false`
      // if(someArbitraryCondition) {
      //   return false;
      // }
    }
  },

isIncrementalMatch

• Optional

A callback used for advanced control of template dependency matching. This determines if a modified file (from a watch/serve rebuild) is relevant to each known full template file. If the callback returns true, the template will be rendered. If the callback returns false, the template will be skipped.

	// some configuration truncated …
  // Called once for each template (matching this custom template’s file extension) in your project.
  isIncrementalMatch: function(modifiedFile) {
    // is modifiedFile relevant to this.inputPath?
    if (this.isFileRelevantToInputPath) {
      // True if they are the same file
      // Or if they are related by any `addDependencies` relationships
      return true;
    }

    // If `modifiedFile` is not a full template (maybe an include or layout)
    // and we have no record of any dependencies for this file, we re-render everything
    if (!this.doesFileHaveDependencies && !this.isFullTemplate) {
      return true;
    }

    // Skip it
    return false;
  },

You can see more advanced override implementations in @11ty/eleventy-plugin-webc and @11ty/eleventy-plugin-vue.

Other pages in Template Languages:

• HTML
• Markdown
  • MDX
• WebC
• JavaScript
  • JSX
  • TypeScript
• Nunjucks
• Liquid
• Handlebars
• Mustache
• EJS
• HAML
• Pug
• Custom